Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

problem with an older AAON unit form 1992

The original service call was for no heat... after checking out the unit the transformer was tripping out when the unit was calling for heat. I narrowed the problem down the a shorted 24v coil on the 2nd contactor for the strip heaters. I pulled the 24volt side of the coil and the unit started up when the power was brought back on with only the 1st and 3rd heaters energized.

Ok so i thought that was the problem... went to the supply house bought a new contactor and came back and installed it. Started the unit and all three contactors for the electric heat pulled in ( delay on the third) and the unit was running fine. Checked my amp draws for the heaters and called it a day.

Two days later the customer called me saying that the unit wasn't working again... so went back to check out the unit and the same problem. The coil had shorted again causes the transformer to trip. Im not sure how long the unit ran before failing again could of been 10 minutes or a day....

So let me give you some of the details of the unit... the transformer has been replaced a few years ago ( i checked to make sure that the VA was the same as the original, checked voltage on the secondary side of the transformer had 24.6 volts but when checking voltage at the coil i only had 23.5 volts... this should be fine right?) The wiring for the heat goes from w1 through 2 limit switchs (in series) then to a to a time delay relay (enables w2 and the third contactor for the electric heat) and the 24 volt coils for the first and second contactors (wired in parallel) with the way they are wired it doesn't make sense to me that only one 24 volt coil would fail??
There has been a problem in the past with contactors failing according to the customer so im not a 100 percent that the problem has ever been fixed. Im looking for things to check for that could be causing the problem... any help would be appreciated thanks

Does this unit have one trans ,,or as I've seen in a lot of aaons two transformers paralleled
?
You sure someone didn't pull two and went back with one ?

Or do you have two ? be absolutely sure of your secondary VA requirements.

You said 14 volts at trans.
Doesn't make sense? i assume you had 24 @ secondary and made a typo.

Is something being energized later causing a big voltage drop?or a faulty component causing one?
Sounds like a voltage drop or undersized transformer .although an undersized trans would probably blow the fuse or burn out before a coil did.

Pull all components "in" and measure your secondary trans amp draw .
If a 75va trans at 24volt that equals an allowable 3.125 max amps to be pulled . VA divided by secondary voltage equals allowed max amps... although I would allow some wiggle room.

.

I suspect a voltage drop thru components ,probably one feeding that particular contactor or undersized trans. If it's all rough how much would the factory limits,contactors,tdr etc cost vs repeat calls.

I also suspect that you might be "leaving " some voltage on that coil when it's not energized . Maybe pulling current thru it but not enough to pull it in.

Also is your primary tapped correctly? is it a multitap trans ,if so check that as well ,although usually if tapped wrong your secondary should be off as well
240 volt supply = trans tapped for 240 not 208 or vise versa,,,,

I am going to check the voltage to the coil of the contactor when i get back with the replacement contactor. I just dont understand why the 1st contactor would be fine and then second would short out with them wired in parallel?? that doesnt make sense right?

So after replacing the contactor im going to make all electric heaters call with the fan and check my amp draw at the secondary side of the transformer... this will tell me if the transformer is the problem?? not to exceed 3.125 amps

Then if i find any voltage from across any components replace them... what is the least amount of voltage to the coil that would work?